Pin psycopg to <2.9 when testing against Django <3.1

As per https://github.com/psycopg/psycopg2/issues/1293

.github/workflows/test.yml

@@ -268,7 +268,7 @@ jobs:
       - name: Install dependencies
         run: |
           python -m pip install --upgrade pip
-          pip install "psycopg2>=2.6"
+          pip install "psycopg2>=2.6,<2.9"
           pip install "${{ matrix.django }}"
           pip install -e .[testing]
           pip install "elasticsearch>=6,<7"